Perhaps something like Markdown Preview? You could create a key binding for the following command and arguments.
{
"keys": ["<your key binding>"],
"command": "markdown_preview",
"args": {
"target": "sublime",
"parser": "markdown"
}
}
You would have to add a context to get it to run with cmd+b, but it's doable. Try out the plugin though and see if it does what you want first.